Results from JULIET (Exercise Training in Obesity-Prone Black and White Women, R01DK49779) show that exercise training improves risk for obesity and diabetes, particularly in African-Americans (AA). In addition, we have shown that intensity of exercise plays a role in affecting the acute phase increase in insulin sensitivity (Si) found 24 after exercise. We have also found that short term positive energy balance can decrease Si significantly. This Competing Continuation is a direct extension of earlier studies by focusing on exercise intensity, energy expenditure (EE), and Si. Since intensity of exercise is more strongly related to increases in Si in AA than in European American (EA) women, it is possible exercise intensity may be more beneficial for AA. Therefore, we plan to evaluate under rigidly controlled energy balance conditions untrained premenopausal AA and EA women before and after 12 weeks of aerobic training. In addition we plan to evaluate their response to a bout of high intensity interval (IE), moderate intensity continuous (CE) or no exercise (NE) after 12 weeks of exercise training. We hypothesize that: 1) EE will be higher for 23 hours following exercise especially following IE;2) Lipid oxidation and Si will be higher (especially for AAs) following the IE after 12 weeks of aerobic training;3) Mitochondrial function and insulin cellular signals will increase but incomplete fatty acid metabolites will decrease following exercise training;4) In trained state, IE will result in increased insulin cellular signaling, mitochondrial oxidation rates of lipid, and reduction in incomplete fatty acid metabolites;5) Changes in lipid oxidation rates, incomplete fatty acid metabolites, and mitochondrial function will be related to changes in Si while changes in muscle damage and sympathetic tone will related to changes in EE. Although we have shown that interval bouts of high-intensity physical activity are well tolerated, a secondary aim of this study is to assess perceived difficulty and muscle pain during the exercise bouts of varying intensity and 27-hours after exercise while performing activities of daily living. We hypothesize that after 12 weeks of aerobic training the IE will not induce more difficulty. This study is innovative in that it is the first systematic evaluation of the acute affects varying exercise intensity has on EE, exercise difficulty, muscle signals and Si in energy balance. The results have potential significance for determining exercise strategies that may be beneficial for the prevention and treatment of obesity and type II diabetes. This is especially important for select, obesity-prone racial groups and for individuals predisposed to diabetes. PUBLIC HEALTH RELEVANCE: This will be the first project to systematically compare the effects of continuous moderate intensity and interval high intensity exercise on resting energy expenditure, insulin sensitivity, skeletal muscle insulin signals, mitochondrial fatty acid oxidation/fatty acid metabolites, mitochondrial function, exercise difficulty, ease of doing activities of daily living, and free living physical activity in African American and European American women. The results have potential significance for determining exercise strategies that may be beneficial for prevention and treatment of obesity and type 2 diabetes, especially for obesity-and-diabetes-prone racial groups.